1. Field of the Invention
The invention relates to the field of power plant technology. It relates to a hybrid blade for thermal turbomachines according to the preamble of patent claim 1.
2. Discussion of Background
Thermal turbomachines, i.e. turbines and compressors, essentially comprise a rotor fitted with moving blades and a stator in which guide blades are suspended. The moving blades and the guide blades in turn essentially comprise an airfoil and a blade root. In order to be able to fasten the blades on the rotor or in the stator, slots are recessed in the stator and on the rotor shaft. The roots of the guide and moving blades are pushed into these slots and locked there.
The fixed guide blades have the task of directing the flow of the gaseous medium to be compressed or of the gaseous medium to be expanded onto the rotating moving blading in such a way that the energy conversion takes place with the best possible efficiency.
It is known to produce blades in one piece from a single material, e.g. from a nickel-base superalloy for gas turbines or from stainless steel for compressors. Such blades are referred to below as conventional blades.
It is also known to produce blades in a hybrid type of construction. In the hybrid type of construction, various materials having different physical properties are combined with one another in order to obtain an optimum design of a blade. Thus, for example, a hybrid rotor blade for a propulsion plant has been disclosed by DE 101 10 102 A1, in which hybrid rotor blade the airfoil trailing edge, which has only an aerodynamic function, is produced from a lightweight construction material, preferably a fiber composite, e.g. a carbon-fiber composite material. The weight of the blade can be advantageously reduced by such a (light) trailing edge. The two airfoil parts (heavy metallic leading edge and light trailing edge made of fiber composite material) are connected by adhesive bonding or riveting.
A similar solution is described in WO 99/27234. A rotor having integral blading, in particular for a propulsion plant, is disclosed there, rotor blades being arranged on said rotor circumferentially, the rotor blades having, for reducing vibrations, a metallic blade root, a metallic airfoil section, which forms at least part of the blade leading edge and the adjoining region of the blade surface, and an airfoil made of fiber-reinforced plastic. Here, too, the airfoil of plastic is fastened to the metallic airfoil section by adhesive bonding/riveting or by clamping.
This known prior art has the following disadvantages. On the one hand, said types of fastening do not withstand high loads over a long period; on the other hand, the fiber-reinforced plastics can be used only within certain temperature ranges, so that these known technical solutions are in particular suitable only for propulsion plant technology. In addition, the characteristic of the airfoil (mechanical properties, oxidation resistance, friction properties) is changed relative to the airfoils made of a single material, a factor which may have an adverse effect on the operating behavior of the machine.
Furthermore, EP 0 513 407 B1 discloses a turbine blade which is made of an alloy on the basis of a dopant-containing gamma titanium aluminide and comprises an airfoil, a blade root and possibly a blade shroud band. During the production of this blade, the cast body is partly heat-treated and hot-worked in such a way that the airfoil subsequently has a coarse-grained structure which leads to high tensile and creep strength, and that the blade root and/or the blade shroud band has a fine-grained structure which leads to increased ductility compared with the airfoil. However, weight reductions compared with conventional blades cannot be realized in this way.